Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and

Transcription

1 Copyright is owned by the Author of the thesis. Permission is given for a copy to be downloaded by an individual for the purpose of research and private study only. The thesis may not be reproduced elsewhere without the permission of the Author.

2 Design and Characterisation of an Open Source Pyrolyser for Biochar Production A thesis presented in partial fulfilment of the requirements for the degree of Master of Engineering In Chemical and Bioprocess Engineering At Massey University, Palmerston North New Zealand Rhonda Bridges 2013

3

4 ABSTRACT An open source field-scale batch pyrolyser was designed and constructed to produce biochar, which is the solid residue formed when biomass thermally decomposes in the absence of oxygen. The design approach was focused on simplicity for the intended target user, a hobby farmer. This is achieved in a batch process, where temperature ramp rates, gas flows and the end-point are controlled. Solids handling is only required at either end of the process. LPG is used as the initial heating source and later as the ignition source when pyrolysis gases are recycled. A mathematical model formulation of the process was developed to predict the proportions of products produced as well as the time taken to achieve complete pyrolysis. Reaction kinetics are complex and not fully understood. In this model, simplifications were taken to provide guidelines for the reactor design as well as the effects of moisture on the process efficiencies. The quality performance of the open source pyrolyser was determined by comparing its biochar to that produced in a lab scale gas fired drum pyrolyser. Parameters varied on the lab drum pyrolyser were highest treatment temperature in the range 300 to 700 C, sample size, moisture content and grain direction for Pinus radiata. The properties that were investigated are elemental composition (C, H, N, S), proximate analysis (moisture, volatile matter and fixed carbon) and char yield (% wt/wt). The ash content was determined by residue on ignition. For the lab scale experiments, it was found with increasing peak temperature that yield, volatile matter and hydrogen to carbon ratio decrease. Yield was unaffected by moisture, size and grain direction. The design of the pilot reactor followed the principle observed with particle size that, in order to get maximum residence time of the vapour and tar in the reactor, the reactor was designed with a perforated core so that the vapours have a tortuous path of travel. This design also meant that heat and mass transfer occurred in the same direction, from the outer wall to the perforated core. In comparison to the lab scale pyrolyser, the same trends were observed in regards to temperature. High yields of 29.7 wt % and 28.8 wt % were obtained from wood with an initial moisture content of 21.9 wt % and 60.4 wt % respectively, confirming yield is unaffected by moisture. Mass and energy balances were conducted on both the lab scale and pilot scale pyrolysers. For every kilogram of carbon in LPG used on the lab scale pyrolyser, an average of 0.25 kilograms of carbon is produced at 700 C. Based on the optimum run for the pilot scale, for every kilogram of carbon in LPG used, 2.6 kilograms of carbon is produced at 700 C. i

5 ii

6 ACKNOWLEDGEMENTS Firstly, I would like to thank my supervisors Jim Jones and Tony Paterson for their time and support throughout this project. I would also like to thank Llewelyn and Judith Richards who were part benefactors of this project. Sadly, Llewelyn passed away prior to the completion of this project. This project has been very beneficial to me and allowed me to put engineering theory into practice and has enabled me to have a greater understanding of the heat and mass transport processes and engineering design. Thanks to the Massey University technical staff that have helped when equipment is broken, cleared space for my wood chips and in general have assisted with my many requests. iii

7 iv

8 TABLE OF CONTENTS 1.0 INTRODUCTION LITERATURE REVIEW Importance of Biochar and Applications Biochar Production by Pyrolysis... 4 Composition Types of Pyrolysis Slow Pyrolysis... 5 Fast Pyrolysis Factors Affecting Pyrolysis Products and Composition Temperature... 6 Heating Rate... 7 Moisture... 8 Vapour Residence Time / Soak Time... 8 Pressure Pyrolysis Reaction Kinetics Models Shafizadeh, Mok et al., Koufopanos et al., Miller and Bellan, Fantozzi et al., Functional-Group, Depolymerisation, Vaporisation and Cross-linking (FG-DVC) Models Neves, Summary Heat and Mass Transfer Heat Transfer...14 Mass Transfer...16 v

9 Single Particle...16 Bed of Particles Reactors Heat Source Wood Properties...19 Water Materials of Construction Summary Aims PYROLYSER DESIGN CRITERIA Overview Batch versus Continuous Pyrolysis Particle Size and Loading/Unloading Start-up Drying Energy Efficiency Non-thermal Energy Requirements Running Costs Cleaning Safety MATHEMATICAL MODELLING OF THE PYROLYSIS REACTION Introduction Model Objectives Conceptual Model Development Assumptions Model Formulation Heat Transfer Across the Boundary...35 Heat Transfer within the Wood...36 Heat Transfer at the Centre...38 vi

10 Pyrolysis Primary Reaction Kinetics...38 Heat Balance for Evaporation of Water Finite Difference Solution For the surface of the reactor...41 For within the reactor...43 For the centre of the reactor Numerical Error Checking Mathematical Solution Conclusions PYROLYSER DESIGN SPECIFICATION Process Description Functional Description Base Frame...50 Loading and Unloading...50 Internal Chamber...51 External Wall / Outer Chamber...52 Fuel Supply...52 Burner...52 Combustion Chamber...53 Flue...53 Tertiary Air...54 Flare...54 Overflow Vent...55 Tar Collection Plate...56 Insulation...57 Monitoring / Measurement Systems Design Specifications Control Description Control Strategy and Testing Procedure Process and Instrumentation Diagram...71 vii

11 5.7 Hazard and operability analysis (HAZOP) CHARACTERISATION Objective Methodology for Lab Scale Pyrolyser Methodology for Field Scale Pyrolyser ANALYSIS TECHNIQUES Biomass Sample Preparation Biochar Sample Preparation Char Yield Biomass Constituent Analysis Porosity and Density Proximate Analysis Residue on Ignition (ROI) Elemental Analysis Statistical Analysis FEEDSTOCK LAB SCALE PYROLYSER RESULTS Biochar Composition Biochar Yield...96 Particle Density...98 Proximate Analysis...99 Elemental Analysis Heating Value Carbon Efficiency PILOT SCALE OPERABILITY AND CHARACTERISATION Discussion of the Results and Operating Conditions for Run Discussion of the Results and Operating Conditions for Run Discussion of the Results and Operating Conditions for Run Discussion of the Results and Operating Conditions for Run Discussion of the Results and Operating Conditions for Run viii

12 10.6 Discussion of the Results and Operating Conditions for Run Operation Summary DISCUSSION OF PILOT RESULTS AND COMPARISON TO LAB RESULTS Yield Biochar composition Carbon Footprint Energy Efficiency Residence Time Heating Rate CONCLUSION RECOMMENDATIONS REFERENCES ix

13 List of Figures Figure 2.1: The effects of temperature on charcoal properties (Antal Jr & Grønli, 2003)... 7 Figure 2.2: One-component mechanism of primary wood pyrolysis (Shafizadeh & Chin, 1976) Figure 2.3: A detailed mechanism for cellulose pyrolysis (Mok & Antal, 1983) Figure 2.4: Kinetic model for the pyrolysis of lignocellulosic material (Koufopanos, et al., 1991) Figure 2.5: Generalised reaction kinetics for cellulose, hemicellulose and lignin (Miller & Bellan, 1997) Figure 2.6: Simplified biomass pyrolysis scheme (Fantozzi, et al., 2007b) Figure 2.7: Thermal degradation of a solid biomass particle under inert atmosphere: drying, primary pyrolysis and secondary pyrolysis Figure 2.8: Pyrolysis of a biomass particle Figure 4.1: Conceptual diagram showing the heat and mass transfer considered in this model as an expansion from the reactor Figure 4.2: The finite difference grid used for pyrolysis Figure 4.3: Temperature profile for conduction through wood Figure 4.4: Conduction and evaporation in wood heated to 700 C Figure 4.5: Conduction, evaporation and reaction of wood pyrolysed at 700 C Figure 5.1: Basic block diagram of the process Figure 5.2: Base frame Figure 5.3: Mechanism for loading and unloading Figure 5.4: Process and instrumentation diagram Figure 6.1: Top view of pyrolyser wood holders Figure 6.2: On the left, a front view of the sampling points, on the right, the sampling points when the reactor is in the horizontal unloading position x

14 Figure 9.1: Raw data including the variables size, moisture, grain direction and peak temperature against yield Figure 9.2: Raw data from the preferred model Figure 9.3: The effect of peak temperature on bulk density of pine pyrolysed under various conditions Figure 9.4: Volatile matter as a function of peak temperature and particle size Figure 9.5: Fixed carbon as a function of peak temperature and size Figure 9.6: Fitted general linear model for ash content on peak temperature, size moisture and grain direction. S is small, M is medium, L is large, G is with the grain and AG is against the grain Figure 9.7: The effect of ash content on peak temperature and particle size Figure 9.8: Ash predicted versus ash experimental on a dry basis Figure 9.9: The effect of peak temperature against Nitrogen; the average nitrogen percentage in unpyrolysed Pinus radiata is Figure 9.10: The effect of peak temperature on size and carbon; the average carbon percentage in unpyrolysed Pinus radiata is 51.3 % Figure 9.11: The effect of size and peak temperature on hydrogen; the average hydrogen percentage in unpyrolysed Pinus radiata is Figure 9.12: van Krevelen diagram of atomic H/C vs O/C ratios for biochar derived from Pinus radiata Figure 9.13: Atomic ratios of H/C vs O/C for carbonised coal (Bituminous and semianthracite), cellulose (van Krevelen, 1950), the Pinus radiata feedstock used in the experiments and the resulting biochar produced from various operating conditions Figure 9.14: The effect of peak temperature on H/C ratios and O/C molar ratios respectively Figure 9.15: The effect of peak temperature and size on the higher heating value Figure 9.16: Conversion of Carbon in Feedstock to Biochar on an Ash-free Dry Basis Figure 9.17: Production Efficiency Figure 10.1: Temperature and LPG profile for Run Figure 10.2: Temperature profile for Run xi

15 Figure 10.3: LPG profile as a function of time and temperature for Run Figure 10.4: Temperature profile for Run Figure 10.5: Temperature and LPG profile for Run Figure 10.6: Temperature and LPG profile for Run Figure 10.7: Temperature profile for Run Figure 10.8: Temperature profile for Run Figure 10.9: Temperature and LPG profile for Run Figure 10.10: Temperature profile for Run Figure 11.1: Burner location in relation to the sampling points Figure 11.2: Comparison of H/C and O/C ratios between the lab and pilot scale reactors. 143 Figure 11.3: The effect of sample position on the hydrogen content of the char in the pilot scale reactor. Note: B = Bottom, T = Top. The number refers to the location in which the sample was collected. Front refers to the front side of the reactor which is on the same side as the burners xii

16 xiii

17 List of Tables Table 5.1: Estimates for design Table 5.2: Control scenarios Table 5.3: Set-point descriptions Table5.4: Time counters Table 5.5: Initial set-points Table 5.6: List Table 6.1: Variables tested during lab scale pyrolysis Table 6.2: Parameters tested on the pilot scale reactor Table 6.3: Soak times Table 7.1: Reference standard for elemental analysis Table 7.2: Coal50 procedure inputs Table 8.1: Constituent analysis of the Pinus radiata feedstock used in the lab scale experiments (Dry basis) Table 8.2: Constituent analysis of the Pinus radiata wood chips used in the pilot scale experiments (Dry basis) Table 8.3: Proximate analysis of the Pinus radiata feedstock used in the lab scale experiments (Dry basis) Table 8.4: Proximate analysis of the Pinus radiata wood chips used in the pilot scale experimental (Dry basis) Table 8.5: Elemental analysis of pinus radiata (Total basis) Table 8.6: Residue on ignition analysis for ash determination (Dry basis) Table 10.1: Proximate analysis results for Run 1 on dry basis Table 10.2: Elemental analysis results for Run 1 on dry basis Table 10.3: Proximate analysis results for Run Table 10.4: Elemental analysis of the sampling points for Run xiv

18 Table 10.5: Proximate analysis results for Run Table 10.6: Elemental analysis results for Run Table 10.7: Proximate analysis for Run Table 10.8: Elemental analysis for Run Table 10.9: Proximate analysis for Run Table 10.10: Elemental analysis for Run Table 10.11: Proximate analysis results for Run Table 10.12: Elemental analysis results for Run Table 11.1: Summary of the biochar yields obtained from the six runs Table 11.2: Summary of the fixed carbon yield results for all runs Table 11.3: Char composition summary for the experiments (Dry basis) Table 11.4: Carbon footprint of the process Table 11.5: Summary of input values for energy efficiency determination Table 11.6: Summary of calculated efficiencies Table 11.7: Residence time Table 11.8: Heating rate Table 14.1 Reactor Volume... B-3 Table 14.2: Summary of system inputs for excess air requirements... B-4 Table 14.3: The effect of flue temperature on the secondary air flow rate... B-14 Table 14.4: Radiation heat losses from the reactor sides... B-21 Table 14.5: Summary of conduction heat losses from the reactor for a lagging thickness of 0.1m... B-29 Table 14.6: Summary of system inputs for explosion vent design... B-30 Table 14.7: Oxygen flow rate as a function of temperature for the 6 kw and 23 kw burners B- 32 Table 14.8: Pyrolysis Gas Composition... B-32 xv

19 Table 14.9: Tar Composition... B-33 Table 14.10: Expansion coefficients for mild steel and stainless steel... B-34 Table 14.11: Summary of expansion from different reactor components... B-35 Table 14.12: Experimental Results... C-2 Table 14.13: Elemental Analysis Results... C-4 Table 14.14: Proximate Analysis Results on a Dry Basis... C-8 xvi